1. Field of the Invention
The present invention relates to a method of fabricating a liquid crystal display device, and more particularly, to a method of forming a color filter layer for a liquid crystal display device.
2. Discussion of the Related Art
In general, a liquid crystal display device uses optical anisotropy and polarization properties of liquid crystal molecules to produce an image. For instance, the orientation of the liquid crystal molecules can be aligned in a specific direction controlled by an applied electric field. As the applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy of the liquid crystal, the refraction of incident light on the liquid crystal molecules also changes depending on the alignment direction of the liquid crystal molecules. Thus, by properly controlling an electric field applied to a group of liquid crystal molecules in respective pixels of a liquid crystal display device, a desired image can be produced by diffracting light.
There are many types liquid crystal displays (LCDs) and one of such types is an active matrix liquid crystal display (AM-LCD) having a matrix of pixels. AM-LCDs are the subject of significant research and development because of their high resolution and superiority in displaying moving images. In general, each of the pixels in an AM-LCD has a thin film transistor (TFT) and pixel electrode.
FIG. 1 is a schematic exploded perspective view of a twisted nematic (TN) mode liquid crystal display device according to the related art. In FIG. 1, a liquid crystal display device 10 includes a first substrate 20, a second substrate 50 spaced apart from the first substrate 20, and a liquid crystal layer 80 interposed between the first and second substrates 20 and 50. The first substrate 20 includes gate lines 22 and data lines 24. The crossing of the gate lines 22 and data lines 24 defines pixel regions “P” and each of the pixel regions “P” includes a thin film transistor “T.” In addition, the TFT “T” includes a gate electrode 26 connected to the gate line 22, an active layer 28, a source electrode 30 connected to the data line 24, a drain electrode 32 spaced apart from the source electrode 30. A transparent pixel electrode 34 connected to the drain electrode 32 is formed in the pixel region “P.”
The second substrate 50 includes a black matrix 52, a color filter layer 54 and a common electrode 56. The black matrix 52 is formed on the second substrate 50 corresponding to the gate lines 22, the data lines 24 and the TFT “T” on the first substrate 20. The black matrix 52 shields light from exterior and is formed of one of an opaque metal and an opaque resin. The color filter layer 54 includes red, green and blue sub-color filters 54a, 54b and 54c alternately disposed. Each sub-color filter corresponds one of the pixel regions “P” and is formed by coating, exposing and developing photosensitive resin.
A first linear polarizing plate 85 having a first polarization axis “C1” is formed outside the first substrate 20 and a second linear polarizing plate 90 having a second polarization axis “C2” perpendicular to the first polarization axis “C1” is formed outside the second substrate 50.
A longitudinal electric field is induced perpendicularly between the pixel electrode 34 and the common electrode 56 by voltages applied to the pixel electrode 34 and the common electrode 56. Such an electric field changes the alignment of the liquid crystal layer 80, thereby changing light transmittance of the liquid crystal layer 80. Thus, as light passes through the liquid crystal layer 80 and the color filter layer 54, desired color images are obtained.
The color filter layer 54 may be formed by various methods including, for example, an electro-deposition method, a dyeing method and a pigment dispersion method. In the electro-deposition method, a color filter layer is formed on an electrode using an electrochemical reaction. The electro-deposition method has superiority in large-sized LCD devices and a low consumption of materials. However, the color filter layer formed through the electro-deposition method has a great deviation in property according to process condition. In the dyeing method, a color filter layer is formed by dyeing a dyeable resin. The color filter layer formed through the dyeing method has low reliability for ultraviolet (UV) light and chemicals. Accordingly, the pigment dispersion method is more commonly used. In the pigment dispersion method, a color filter layer is formed by coating and exposing a material where polyimidic pigments are dispersed. The pigments are insoluble in the solvent.
FIGS. 2A to 2D are schematic perspective views showing a process of forming a color filter substrate for a liquid crystal display device according to the related art.
In FIG. 2A, a black matrix 52 is formed on a substrate 50 having red, green and blue pixel regions “PR,” “PG” and “PB” corresponding to pixel regions on a thin film transistor substrate (not shown) facing the substrate 50. The black matrix 52 is formed of one of chromium (Cr) and opaque resin. A double layer of chromium/chromium oxide (Cr/CrOx) also can be used for forming the black matrix 52.
In FIG. 2B, a red resist layer 53 is formed on the entire surface of the substrate 50 having the black matrix 52 by coating a photosensitive color resist including red pigment. The photosensitive color resist is a negative type photoresist where a portion exposed to light remains after development. Even though not shown in FIG. 2B, a mask having a transmissive portion and a shielding portion is disposed over the red resist layer 53, such that the transmissive portion corresponds to the red pixel region “PR.” Light is then irradiated onto the red resist layer 53 through the transmissive portion of the mask and then the red resist layer 53 is developed.
In FIG. 2C, a red sub-color filter 54a corresponding to the red pixel region “PR” is formed on the black matrix 52 after the red resist layer 53 shown in FIG. 2B is developed. The red sub-color filter 54a then is cured with heat in a subsequent process.
In FIG. 2D, green and blue sub-color filters 54b and 54c are formed to correspond to the green and blue pixel regions “PG” and “PB” by repeating a process similar to the process shown in FIGS. 2A to 2C with a photosensitive color resist including respective color pigment. Thus, a color filter layer 54 including the red, green and blue sub-color filters 54a, 54b and 54c is formed.
The above-described steps of exposing and developing are generally referred to as a photolithographic process. For instance, the process may include providing a mask on a resist layer and exposing the resist layer through a mask using an exposing apparatus. The exposing apparatus can be a lens projection exposing device where a resist layer is exposed by sequentially moving a substrate and a mask in a stepping manner to obtain multiple sub-color filters on the substrate. In other words, the mask and a platform having the substrate thereon are moved sequentially with respect to the exposing device to expose the resist layer and to form a plurality of patterns on the substrate with only one mask. However, since one large pattern is formed using several masks having a pattern corresponding to a portion of the large pattern, each mask for the stepping method has a margin for misalignment. This margin reduces an effective area of the mask. Moreover, as patterns become more miniature, the exposing apparatus including lenses and photo becomes more expensive.